Void Ratio Dependency Research Articles

Development of a Multiphase Numerical Modeling Approach for Hydromechanical Behavior of Clay Embankments Subject to Weather-Driven Deterioration

Clay embankments used for road, rail, and flood defense infrastructure experience a suite of weather-driven deterioration processes that lead to a progressive loss of hydromechanical performance: micro-scale deformation (e.g., aggregation and desiccation), changes in soil-water retention, loss of strength, and macro-scale deformation. The objective of this study was to develop a numerical modeling approach to simulate the construction and long-term, weather-driven hydromechanical behavior of clay embankments. Subroutines within a numerical modeling package were developed to capture deterioration processes: (1) strength reduction due to wet-dry cycles; (2) bimodality of the near-surface hydraulic behavior; (3) soil-water and soil-gas retentivity functions considering void ratio dependency; and (4) hydraulic and gas conductivity functions considering void ratio dependency. Uniquely, the modeling approach was comprehensively validated using laboratory tests and nine years of field measurements from a full-scale embankment. The modeling approach captured the variation of near-surface soil moisture and matric suction over the monitored period in response to weather cycles. Further, the developed model approach could successfully simulate weather-driven deterioration processes in clay embankments. The model predictions manifested the ability of the modeling approach in capturing deterioration features such as irrecoverable increases in void ratio and hydraulic permeability near surface. The developed and validated numerical modeling approach enables forecasting the long-term performance of clay embankments under a range of projected climate conditions.

Estimations of water retention curves for fine-grained soils and tailings using their fractal particle size distributions

A water retention curve (WRC) enables the quick determination of suction in a soil or tailings using the gravimetric water content or degree of saturation. However, establishing the WRC is reliant on time consuming laboratory testing. Here an empirical method is presented that allows the WRC to be estimated quickly, including its void ratio dependency, by relating the fractal and other characteristics of the soil or tailings particle size distributions (PSD) to the parameters which define the WRC. The defining parameters, from a fractal mathematics viewpoint, depend on the particle and pore surface areas and shapes as well as their fractal dimensions. More generally, the clay percentage must also have an important role as it has a major influence on these properties, especially the surface areas. Twelve WRCs for soils and tailings were compiled and compared and used to establish the new empirical expressions. Estimated WRCs using these new empirical expressions compare well with those fitted directly to experimental results. Guidelines are also given on how to quantify suction’s contribution to strength using the location of the hydraulic state on the WRC, adding to the practical appeal of this work.

Triaxial test on water absorption compression of unsaturated soil and its soil-water-air-coupled elastoplastic finite deformation analysis

Triaxial tests and numerical simulations were conducted to determine the water absorption compression of unsaturated soil. In the triaxial tests, unsaturated silt specimens under the same initial condition were prepared and the processes of different suction reduction, isotropic consolidation, and exhausted-drained shearing were applied to the specimens. The numerical simulations used a soil-water-air coupled finite deformation analysis code with a newly incorporated elastoplastic constitutive model. The degree of saturation was introduced to the internal state variables of the yield function of the SYS Cam-clay model and a soil-water characteristic model with void ratio dependency. Then, the triaxial test was treated as an initial and boundary value problem, and the series of processes in the experiments were simulated using a single set of material constants and initial values. The new findings are as follows:1)The loading criterion and the classification of loading conditions considering change in saturation degree can be specified in the elastoplastic constitutive model for unsaturated soil.2)Water absorption compression behavior was observed in the experiments not only under suction reduction but also under constant suction condition. This water absorption compression under constant suction condition was explained as a saturation-increase behavior due to volume compression using the soil-water characteristic model with void ratio dependency.3)By considering the triaxial test as an initial and boundary value problem, the temporal change in the volume and the amount of water absorption/drainage were able to be reproduced.

Predictive potential of Perzyna viscoplastic modelling for granular geomaterials

SummaryThis paper reappraises Perzyna‐type viscoplasticity for the constitutive modelling of granular geomaterials, with emphasis on the simulation of rate/time effects of different magnitude. An existing elasto‐plastic model for sands is first recast into a Perzyna viscoplastic formulation and then calibrated/validated against laboratory test results on Hostun sand from the literature. Notable model features include (1) enhanced definition of the viscous nucleus function and (2) void ratio dependence of stiffness and viscous parameters, to model the pycnotropic behaviour of granular materials with a single set of parameters, uniquely identified against standard creep and triaxial test results. The comparison between experimental data and numerical simulations points out the predicative capability of the developed model and the complexity of defining a unique viscous nucleus function to capture sand behaviour under different loading/initial/boundary and drainage conditions. It is concluded that the unified viscoplastic simulation of both drained and undrained response is particularly challenging within Perzyna's framework and opens to future research in the area. The discussion presented is relevant, for instance, to the simulation of multiphase strain localisation phenomena, such as those associated to slope stability problems in variably saturated soils.

Hysteretic Model for the Evolution of Water Retention Curve with Void Ratio

AbstractThis paper presents a model for the evaluation of the void ratio dependency of water retention curve (WRC) in deformable porous media. Models currently available in the literature for this ...

Development of global correlation models between in situ stress-normalized shear wave velocity and soil unit weight for plastic soils

Shear wave velocity (Vs) in geo-materials is strongly dependent on factors such as stress state, void ratio, and soil structure. Stress-dependency and void-ratio dependency can be represented by the equations [Formula: see text] and Vs= a(e)b(where α and a are material constants; exponents β and b represent the sensitivity of stress and the void dependent effect, respectively; [Formula: see text] is effective confining stress; e is void ratio), respectively. To consider the effect of soil disturbance and stress relief in geo-materials, shear wave velocity is often required to be normalized by adopting the site-specific model parameters (β or b). Based on a special in situ database compiled from 156 well-documented test sites that include various geo-materials, this study presents (i) the apparent relationships of the model parameters α and β for all soil and rock materials as well as a and b for all soil materials, (ii) new global correlations between soil unit weight and two types of stress-normalized shear wave velocities (Vs1and Vsn), instead of the conventional Vs– soil unit weight relationship for clays, and (iii) the best-fitted multi-regression models between soil unit weight and site-specifically normalized shear wave velocity as well as the plasticity index for plastic soils. Moreover, this study presents the importance of site-specific stress normalization (Vsn) in creating a better correlation model. The proposed relationships offer first-order assessments of soil unit weight within the ranges of available data, which are also approximately guided by a hyperbolic unit weight model with depth.

A void ratio dependent water retention curve model including hydraulic hysteresis

Past experimental evidence has shown that Water Retention Curve (WRC) evolves with mechanical stress and structural changes in soil matrix. Models currently available in the literature for capturing the volume change dependency of WRC are mainly empirical in nature requiring an extensive experimental programme for parameter identification which renders them unsuitable for practical applications. In this paper, an analytical model for the evaluation of the void ratio dependency of WRC in deformable porous media is presented. The approach proposed enables quantification of the dependency of WRC on void ratio solely based on the form of WRC at the reference void ratio and requires no additional parameters. The effect of hydraulic hysteresis on the evolution process is also incorporated in the model, an aspect rarely addressed in the literature. Expressions are presented for the evolution of main and scanning curves due to loading and change in the hydraulic path from scanning to main wetting/drying and vice versa as well as the WRC parameters such as air entry value, air expulsion value, pore size distribution index and slope of the scanning curve. The model is validated using experimental data on compacted and reconstituted soils subjected to various hydro-mechanical paths. Good agreement is obtained between model predictions and experimental data in all the cases considered.

Hydro-mechanical hypoplastic models for unsaturated soils under isotropic stress conditions

The current work presents two hypoplastic models to predict the hydraulic and mechanical behavior of the unsaturated soils. The hydraulic model is capable to reproduce the observed hysteretic behavior for the drying and wetting processes and incorporates the void ratio dependence. The mechanical model is written in terms of the Bishop effective stress and is limited only to isotropic states. It incorporates a normal consolidation line that depends on the degree of saturation. Both models were developed under the hypoplastic framework and therefore are capable to simulate the observed behavior without defining a yield surface. Simulations of some experiments performed with Pearl clay suggest that the models accurately predict its hydro-mechanical behavior.

Advances in modelling hysteretic water retention curve in deformable soils

Experimental findings on the hysteretic nature of the soil water retention curve, relating the degree of saturation to the matric suction, have generally to be superimposed with the aspects due to the deformability of the soil matrix. Indeed, most state-of-the-art models for retention curves only feature one of these two essential features, that is either capillary hysteresis or void ratio dependency. In an effort to set an advanced comprehensive model for the retention curves, it is proposed to review some recent results of the capillary hysteresis and focus on the elasto-plastic analogy in the degree of saturation versus suction relationship. The paper also contributes to quantifying the effects of mechanical straining on the retention curve on the basis of experimental data from the literature besides those obtained by the authors. The intrinsic shape of the soil water retention curve is first defined, followed by the empiric relationship between air entry value and void ratio. The retention sub-model of a complete constitutive model for unsaturated soils is described, the mathematical formulation being based on kinematic hardening and featuring direct coupling with the mechanical stress-strain module. Model capabilities are assessed on complex retention outlines, displaying the added value of the proposed framework for prediction issues.

Mechanical modeling of MX-80–Development of constitutive laws

In a previous paper [Åkesson, M., Hökmark, H., 2007. Mechanical model for unsaturated MX-80. In: Schanz, T. (Ed.), Theoretical and Numerical Unsaturated Soil Mechanics, Springer Proceeding in Physics, vol. 113, pp. 3–10] a new elastoplastic model for unsaturated swelling clay was presented. The model was formulated as two differential equations, for elastic and plastic strains, respectively. Each equation described a relation between the (air-filled) macro void ratio and the two independent variables stress and (water-filled) micro void ratio. The established concept of swelling pressure and its void ratio dependence were incorporated in the model. To do this, the grain-to-grain contact stress and the contact area had to be considered. In its original form the model only handled one-dimensional problems, e.g. compression tests with uniaxial strain and water uptake test at constant volume with assumed isotropic conditions. This paper describes a first attempt to generalize the model to address multidimensional problems with the intention to integrate it into the framework of Code_Bright. This was made through definition of an expression for the retention properties and through variable substitution. In this way, four functions for the kappa moduli were identified: two elastic and two plastic. These functions provide unique values for each given state. In the used framework of the Barcelona Basic Model (BBM), all strains are treated as elastic, and there is thus no explicit treatment of any deviatoric yield condition. The only remaining parameter outside the new model is the Poisson’s ratio.